Underbalanced Drilling: General Issues

Coiled Tubing for Underbalanced Drilling Coiled tubing is ideally suited for drilling underbalanced. The continuous nature of the coiled tubing string and the exclusive use of a mud motor for drilling provides several advantages over conventional jointed pipe drilling for underbalanced applications: Drilling with a mud motor prevents the formation of key seats, thus eliminating one cause of stuck pipe.Because the coiled tubing string is not rotated, mechanical damage caused by pipe rotation (e.g., pasting of cuttings against the borehole wall) is not an issue.Making connections during conventional jointed pipe operations requires stopping and then re-starting the pumps. The gas and liquid phases separate when pumping stops, and then two-phase circulation resumes once the connection is made and the pump is re-started. This continual cycling results in pressure surging of the wellbore and can lead to overbalanced surging of the reservoir. Additionally, halting the pumps can result in the formation of a cuttings bed. When drilling resumes, the cuttings bed may be gradually ground up into the borehole wall. With coiled tubing UBD, these problems are avoided the drill string is continuous, allowing the bottom hole pressure to be kept consistent, with no forced surging of the reservoir. And with properly designed wellbore hydraulics, the formation of a cuttings bed can be avoided.The continuous pipe provides constant well control and improved worker safety with minimal pipe handling and faster trip times.Coiled tubing has a high resistance to sulfide stress cracking and can be left in the hole as a production string.The electric cable inside the coil provides real time monitoring of down hole information (e.g., annulus pressure, directional data, logging while drilling). Of course, theres are also limitations iinvolved in coiled tubing drilling. If borehole stability problems develop, coiled tubing cannot be rotated. Neither can it withstand the physical abuse that conventional drill collars and drill pipe can. Therefore, in formations prone to sloughing or washing out, coiled tubing should not be the automatic first choice. Due in part to logistical concerns and also due to the current size and weight of coiled tubing, generally, horizontal lengths and hole sizes are smaller than those achieved with larger conventional equipment. Underbalanced Directional Drilling Directional Drilling equipment can impose limitations on underbalanced drilling. Hydraulically operated tools cannot be used in UBD wells, and if a gasified system is used the MWD pulse systems may not work. Certain motors and other directional equipment may be prone to failure as a result of the rubber components becoming impregnated with the gas used. The potential for explosive decompression of rubber components needs to be considered when selecting equipment. The higher Torque and Drag seen in UBD wells (as much as 20%) may also prevent certain trajectories from being drilled underbalanced. The higher torque is caused by the reduced buoyancy combined with the lack of filter cake on the borehole wall. Positive Displacement Motors (PDMs), or downhole motors, are designed for drilling mud and not air, although some have been adapted to air. Currently there is no reliable MWD system that will work consistently with a motor and air drilling, although an Electromagnetic MWD (EMWD) will work in an air environment. When used with a downhole motor, the motor vibrations quickly damage the EMWD because there is no hydraulic dampening of the vibrations from the motor. The EMWD will work with rotary BHAs since the vibration is much less. Even steering tools experience frequent failures from motor vibration; steering tools will work better if mist is added to the air because the fluid helps to damp the vibrations downhole. Well Logging of Underbalanced Holes Most logging tools are sensitive to the type of fluid present at the time of the logging. Depending on the level of underbalance, the wireline tools might have to be run through special lubricators to maintain underbalanced conditions. As technology changes, service companies are continually improving their existing logging tools. Therefore, you should always check with your service company to learn more about the most recent tools available for wells drilled with gas or gasified liquids. Table 1 summarizes the applicability of various logging tools in underbalanced drilling. Table 1: Well logging in air and gasified liquid drilling. Logging TechniqueAir DrillingGasified Liquids DrillingSpontaneous Potential LogNoNoGamma Ray LogYesYesNeutron LogYes (with correction)Yes (with correction)Density LogYes (with correction)Yes (with correction)Resistivity LogNoNoInduction LogYesYesNMR LogYesYesAcoustic LogNoNoTable 1: Well logging in air and gasified liquid drilling Safety in Underbalanced Drilling Because of the large volumes of oil and gas production potential in underbalanced drilling, strict safety procedures have to be implemented during the drilling operation. Potentially explosive and flammable hydrocarbons have to be isolated while they are produced during drilling. In onshore operations, oil is sent to storage facilities while gas is typically flared. In offshore operations, due to limited storage space available, both oil and gas are flared. The presence of hydrogen sulfide (H2S) in produced hydrocarbons poses an extreme danger to the rig personnel. H2S is extremely poisonous and can be fatal even at very low concentrations. If the presence of H2S is anticipated during drilling, the operator must provide: Necessary notice of the proposed operations and hazards.Special training.An emergency contingency plan.H2S resistive material.H2S sensors, warning alarms, portable and fixed air breathing devices. For conducting a safe underbalanced drilling operation, proper personnel training is a key issue. Having the critical manpower helps to avoid unforeseen emergencies. Storage and flaring equipment have to be properly sized according to the anticipated volume of hydrocarbon production. Wind direction should also be taken into consideration while positioning the flare system and the storage facilities. Since the presence of air can cause a corrosive downhole environment, explosions and downhole fires, extreme care must be exercised to avoid combustible mixtures. In order to control the back flow from drillpipe, at least two drillstring floats should be installed on the drill string assembly. Depending on the type of the underbalanced drilling fluid used, more floats may be needed for maximum control and safety. BOP stacks should be designed with extreme care by going through a step-by-step testing for every single system part. They should be tested when installed, once every week and following each repair. The drilling operation should be stopped immediately if pressures exceed the maximum design limits. Underbalanced Well Classification System A classification system developed by the International Association of Drilling Contractors’ Underbalanced Operations Committee was issued in Spring 2001. The main purpose of this classification system is to establish a reference database of wells drilled with different underbalance levels and techniques. The classification system is presented in the form a matrix (Table 1). Table 1: IADC-UBO committee classification matrix for underbalanced wells. The first part of the system identifies the well in terms of associated risk levels, as defined in Table 2. Following the risk level identifier, a sub-classifier is used to define whether the well is drilled underbalanced or with a "low head". The last component of the notation identifies the type of underbalanced drilling technique used (Table 3). Risk LevelIdentification0Performance enhancement only; no hydrocarbon containing zones.1Well incapable of natural flow to surface. Well is "inherently stable" and is low level risk from a well control point of view.2Well is capable of natural flow to surface but enabling conventional well kill methods and limited consequences in case of catastrophic equipment failure.3Geothermal and non-hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.4Hydrocarbon production. Maximum shut-in pressures less than UBD equipment pressure rating. Catastrophic failure has immediate serious consequences.5Maximum projected surface pressures exceed UBD operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences.Table 2: IADC-UBO committee underbalanced well risk level identifiers. UBD TechniqueDefinitionGas DrillingThe overall process of drilling using only gas as the drilling medium. No intentional fluid added.Mist DrillingDrilling with liquid entrained in a continuous gaseous phase. Typical mist systems have less than 2.5% liquid content.Foam DrillingDrilling with a two-phase fluid with a continuous liquid phase generated from the addition of liquid, surfactant, and gas. Typical foams range from 55% to 97.5% gas.Gasified Liquid DrillingDrilling with a gas entrained in a liquid phase.Liquid DrillingDrilling with a single-phase liquid.Table 3: IADC-UBO committee underbalanced drilling technique definition. For example, a vertical section of a geothermal well drilled in a known formation with a low head nitrogen-lightened drilling fluid would be classified as: 3-A-4. Selection of Candidates for UBD An important step in designing an underbalanced drilling operation is to select the appropriate technique and fluid system. As a starting point, recall that underbalanced drilling works to particular benefit in reservoirs that are prone to formation damage,low-pressure or depleted reservoirs,disposal wells. If one or more of the above conditions applies, you should make an initial evaluation to select the appropriate underbalanced drilling technique and fluid type. This evaluation should use conventional drilling applications as a basis for comparison. In some situations, the applicability of underbalanced drilling is not as clear-cut. Such situations might include: Low-pressure reservoirs with fractures.Very high permeability reservoirs.Extremely high-pressure formations. These types of conditions require a more in-depth study of the circumstances and type of the proposed UBD technique. Finally, there are certain conditions under which it is practically impossible to successfully apply UBD methods: Unconsolidated formations.Presence of swelling shale zones.Wellbore instability. Which Underbalanced Technique to Use There are set conditions that determine the applicability of different underbalanced drilling techniques. Table 1 shows the preferred techniques for various drilling conditions. ConditionPreferred UBD TechniqueLow Rate of Penetration (ROP) through hard rock- Dry air.- Mist, if there is slight water inflow.- Foam, if there is heavy water inflow.- Nitrogen or natural gas, if the well is producing wet gas.Lost circulation through the overburden- Aerated mud, if ROP is high or if water sensitive shales are present.- Foam, if wellbore stability is not a problem.Differential sticking through the overburden- Nitrified mud, if gas production is likely.- Aerated mud, if gas production is unlikely and an open surface system is to be used.- Foam, if pore pressure is very low and if the formations are very hard.Formation damage through a soft/medium depleted reservoir- Nitrified brine or crude with string injection or parasite string.- Foam, if the pore pressure is very low and an open surface system is acceptable.Table 1: Preferred UBD technique for different conditions. The following factors have to considered before deciding what type of underbalanced fluid to use: Presence of shales.Water production.Multiple zones with significantly different pore pressure.Possibility of sour gas production.Equipment availability. The first step in selecting a UBD technique is to calculate the expected pressure profile within the wellbore. Borehole pressure limits should be based on the following factors: Pore pressure.Wellbore stability.Tensile failure.Shear failure.Matrix collapse.Chemical stability of shales. It may not always be possible to accurately calculate the formation pore pressure. In such cases, offset well data and wireline logs should be used to obtain estimates. Numerical mechanics modeling and field-testing should be used together in order to define stress fields around the wellbore. Bottomhole pressures should be kept within elastic stability limits defined by shear and tensile failure envelopes. As soon as the wellbore instability and maximum pore pressure limits are determined, you may evaluate the different techniques by using the equivalent circulating densities of underbalanced drilling fluids (Figure 1: Underbalanced drilling fluid gradients) . FIGURE 1 Since pore pressure gradients and formation strength can vary with depth, a limiting borehole pressure envelope should be constructed on top of the pressure gradient curves. The types of drilling fluids that are applicable at different depths can then be determined as shown in Figure 2, where the pore pressure gradient in a depleted interval reduces the maximum tolerable wellbore pressure. When evaluating highly productive formations, a detailed numerical analysis of circulating pressures may be needed to estimate the ideal borehole pressures and production rates that would satisfy underbalanced conditions. Water production makes gas drilling difficult because of mud ring formation. If water flows are encountered or anticipated, operations must be converted over to mist or foam. Also, the use of nitrogen or natural gas for mist drilling generally requires a high gas rate. Gas supply cost thus becomes a critical consideration in designing an underbalanced operation. Mist and foam drilling generate similar volumes of wastewater. Therefore, if water disposal costs prevent the application of mist drilling then foam drilling probably will be also uneconomical. FIGURE 2 Figure 2: “Tolerable Wellbore Pressure” representation example on fluid gradient chart. Multiple zones The presence of more than one permeable zone complicates underbalanced drilling operations, because the circulating pressure must satisfy pressure requirements of all permeable zones exposed to the wellbore. Formation fluids may flow from one permeable zone into another, resulting in an underground blowout. Since it is almost impossible to properly maintain different levels of underbalance at different depths under dynamic drilling conditions, the casing design should be such that the upper zones will be sealed off before penetrating the lower zones. Setting intermediate casing also prevents the possibility of fracturing openhole sections during well killing operations. Hole geometry It is quite difficult to keep uniform foam quality in deep wells, since additional backpressure may be needed to establish the necessary pressure gradient in the wellbore. Larger hole diameters require higher circulating rates for effective hole cleaning. Therefore, high viscosity fluids become more attractive as the hole diameter increases. For hole diameters of 17½ in. [445 mm] and larger, stiffened foams and gasified liquids are recommended for improved cleanup efficiency. Some UBD equipment may be hard to obtain for larger hole diameters. Underbalanced drilling fluid selection Based on their equivalent circulating densities underbalanced drilling fluids are classified into five major categories: Gas (Air, Nitrogen).Mist.Foam.Gasified Liquid.Liquid. The selection of any of these fluids for underbalanced drilling operation can be a complicated process. Factors to consider in this selection process include: Reservoir rock/fluid properties.Fluid compatibility/characteristics.Hole cleaning.Bottomhole assembly.Type of data transmission.Corrosion.Temperature stability.Surface fluid handling and separation.Health and safety considerations.Environmental factors.Availability. According to the UBD fluid guidelines published by the International Association of Drilling Contractors (IADC), drilling fluid selection is conducted in three stages: Stage 1: What is the purpose for implementing underbalanced drilling? Stage 2: What are the types of fluids that can be considered for underbalanced drilling? Stage 3: Which fluid should be used for a given well? Stage 1 The type of underbalanced drilling fluid to be used depends on the overall well objectives. At this stage, the lowest and highest pore pressures as well as the depth of target zone have to be identified. Borehole stability and anticipated production play a crucial role in defining the necessary pressure gradient (equivalent circulating density) to be established by the underbalanced drilling fluid. Surface facilities and solids processing equipment have to be designed based on the expected amount of cuttings carried to the surface. The separation, storage and transmission of produced hydrocarbons while drilling should also be taken into consideration. Stage 2 In selecting the appropriate fluid type, we may start with the IADC classification system for (Table 2) that defines a well's risk level in terms of its performance characteristics. Risk LevelIdentification0Performance enhancement only; no hydrocarbon containing zones.1Well incapable of natural flow to surface. Well is "inherently stable" and is low-level risk from a well control point of view.2Well is capable of natural flow to surface but enabling conventional well kill methods and limited consequences in case of catastrophic equipment failure.3Geothermal and non-hydrocarbon production. Maximum shut-in pressures less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.4Hydrocarbon production. Maximum shut-in pressures less than UBD equipment pressure rating. Catastrophic failure has immediate serious consequences.5Maximum projected surface pressures exceed UBD operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences.Table 2: IADC-UBD committee underbalanced well risk level identifiers. We then need to estimate the maximum and minimum pressure limits for the formation using the pore pressure (maximum pressure limit) and the borehole stability criterion (minimum pressure limit) as described above. Figure 1 shows how these values are plotted in terms ECD for different underbalanced drilling fluids. Table 3 presents IADC classification of underbalanced drilling fluids and their corresponding equivalent circulating densities. Table 3: Underbalanced drilling fluid classification Selection of the drilling fluid also controls equipment requirements. Table 4 provides a brief summary of equipment requirements as a function of underbalanced drilling fluid group. FLUID GROUPEQUIPMENT REQUIREMENTSGas Drilling- For Air Drilling: Compressors, boosters, mist/foamer pump, blooie line, rotating head/diverter, flare/flame, drill string floats.- For Nitrogen Drilling: Cryogen tanks heaters – OR – membrane nitrogen generators, boosters, mist/foamer pump, blooie line, rotating head/diverter, flare/flame, drill string floats.- For Natural Gas Drilling: Pipeline / gas source, compressors, boosters, mist/foamer pump, blooie line, rotating head/diverter, flare/flame, drill string floats.Mist DrillingSource of gas, small injection pump, compressors, boosters, mist/foamer pump, blooie line, rotating head/diverter, flare/flame, drill string floats.Foam DrillingSource of gas, compressors, boosters, foam generator, blooie line, rotating head/diverter, flare/flame, special metering equipment, defoaming tank and pump, drill string floats.Gasified Liquid DrillingGas/liquid separator, compressors, boosters, flare line, rotating head/diverter, flare/flame, drill string floats.Liquid Drilling- For Oil Based Drilling: Rotating head/diverter, drill string floats, cuttings disposal.- For Emulsion Drilling: Rotating head/diverter, drill string floats, cuttings disposal.- For Water Based Drilling: Rotating head/diverter, drill string floats. Table 4: Equipment requirements for various underbalanced drilling fluid groups. Stage 3 After collecting all the necessary information listed in the previous two stages, we select the best drilling fluid based on: Compatibility of drilling fluids with formation lithology and fluids.Hole cleaning requirements.Degree of corrosion risk both on surface and downhole equipment.Local safety and regulatory compliance issues.Availability and cost of drilling fluids.Availability and cost of drilling equipment. Along with the above listed factors, processing and disposal of cuttings and associated fluids also causes additional expense. You should calculate the incremental cost for all applicable drilling fluids versus the savings in drilling time before making the final decision. Clearly, the overall goal is to select the drilling fluid that provides optimal incremental drilling performance at minimum incremental cost. Managed Pressure Drilling (MPD) Managed pressure drilling (MPD) is an adaptive drilling process designed to precisely control the annular pressure profile throughout the well. The main idea is to create a pressure profile in the well to stay within close tolerances and close to the boundary of the operation envelope defined by the pore pressure, hole stability envelope and fracture pressure. MPD uses different techniques to mitigate the risks and costs associated with drilling wells by managing the annular pressure profile. These techniques include controlling backpressure, fluid density, fluid rheology, annular fluid level, circulating friction, and hole geometry in any combination The International Association of Drilling Contractors (IADC) has defined MPD further by creating two categories. Reactive MPD includes drilling programs that are tooled up with at least a rotating control device (RCD), choke, and perhaps drill string float to safely and efficiently deal with problems that could occur downhole. Proactive MPD includes designing a casing, fluids and open hole program that precisely manages the wellbore pressure profile. This category of MPD can offer the greatest benefit to the offshore drilling industry as it can deal with unforeseen problems before they occur. While it uses many of the same tools that have been designed for UBD operations, MPD is fundamentally different from UBD in that it involves drilling overbalanced. While UBD is mainly used to prevent damage to the reservoir, the purpose of MPD is to solve drilling problems. UBD allows influx of formation fluids by drilling with the pressure of the fluid in the wellbore lower than the pore pressure. MPD is based on maintaining the pressure between the pore pressure and the fracture pressure of the reservoir. It is set up to handle the influx of fluids that may occur while drilling but does not encourage influx. In summary, UBD is reservoir-issue related while MPD is drilling-issue related. Assessment 1. Spontaneous Potential (SP) Log can be used in air drilling operations. A .TrueB .False ✔ 2. Induction logging can be used only in ___________ operations. A .air drillingB .liquid drillingC .gasified liquid drillingD .A, B and C ✔ 3. It is NOT possible to run the acoustic logging tool in air drilling operations. A .True ✔B .False 4. Which of the following logging techniques can NOT be used in gasified liquid drilling operations? A .Gamma ray logB .NMR logC .Resistivity log ✔D .Density log 5. Which of these is NOT necessary if the presence of H2S is anticipated during underbalanced drilling? A .Special trainingB .Presence of a fire truck at the drilling site ✔C .An emergency contingency planD .Portable and fixed air breathing devices 6. At least _______ drillstring floats should be installed on the drillstring assembly in order to control the back flow from drillpipe. A .1B .2 ✔C .4D .5 7. The underbalanced well classification system is developed by: A .The Society of Petroleum EngineersB .The Society of Professional Well Log AnalystsC .The American Petroleum InstituteD .The International Association of Drilling Contractors ✔ 8. What is the highest risk level defined according to the classification matrix for underbalanced wells? A .5 ✔B .10C .50D .100 9. What risk level would you assign to a well which has some hydrocarbon production and maximum shut-in pressures less than UBD equipment pressure rating? A .1B .4 ✔C .5D .10 10. According to this classification system, how would you code a well, which is not capable of natural flow drilled with low head drilling liquids? A .0-B-5B .1-A-5 ✔C .0-A-4D .1-A-4 Recommended for You Dry Air Drilling Nitrogen Drilling Natural Gas Drilling Mist Drilling Foam Drilling | What is Foam Drilling? Gasified Liquids Flow Drilling Snub Drilling

Logging Technique	Air Drilling	Gasified Liquids Drilling
Spontaneous Potential Log	No	No
Gamma Ray Log	Yes	Yes
Neutron Log	Yes (with correction)	Yes (with correction)
Density Log	Yes (with correction)	Yes (with correction)
Resistivity Log	No	No
Induction Log	Yes	Yes
NMR Log	Yes	Yes
Acoustic Log	No	No

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